The invention is in the field of anechoic chambers used principally for testing electronic equipment for wave emissions.
In the design of anechoic chambers according to present techniques and technology, the level of performance of the chamber is almost strictly a function of cost, assuming the design is state-of-the-art. A typical chamber at today's prices runs several hundred thousand dollars, a cost which is largely proportional to the interior surface area of the chamber which must be lined with expensive absorber material. Assuming proper chamber and absorber design, increased performance of a box-shaped chamber can be achieved routinely by enlarging the chamber and thickening the absorber material. However, the cost will go up geometrically or even exponentially as a function of improved performance.
A cubicle chamber is fairly effective in absorbing radiation and preventing its reflection from the walls onto the measuring instruments. However, inasmuch as the wave front from the source to the receiver is a straight line, the use of a cube rather than a longer, narrower chamber would seem to be a poor design in view of the high cost of lining the entire interior of the cube with absorber material.
As the chamber is drawn out into a longer, narrower design, increasing the so-called "aspect ratio," a chamber of the appropriate length can be made at a considerable cost savings. The tradeoff for the cost savings, however, is a substantial drop in chamber performance. This drop is a result of the high reflectivity of radiation off the chamber walls when the radiation strikes at a relatively low angle of incidence. Radiation striking the absorber material at a right angle, or within about 30.degree. of a right angle, yields the highest level of absorption, which can be 99% for most frequencies, and up to 99.9% in some instances. Even at the 45.degree. level, which would be the worst case in a cubicle chamber, performance loss is not critical.
However, as the aspect ratio of the chamber grows to about 2.5, the angle of incidence of radiation reflected directly onto the receiver from the sidewalls, roof, and ceiling, drops below 30.degree.. At these low angles of incidence, there is a drastic drop off of absorbtivity.
There is thus the need for an anechoic chamber which utilizes a relatively high aspect ratio to avoid the high cost of lining a cube with absorber material, but which also causes radiation incident on the sides of the chamber to impinge at angles much greater than 25.degree.-30.degree., so that the performance is moved up the performance curve into the area where reflectivity remains on the order of magnitude of the reflectivity from a perpendicular angle of incidence.